Stars seem brighter this time of year

As seen during Northern Hemisphere winter (or Southern Hemisphere summer), the stars seem brighter. Why? It’s partly because – on December, January and February evenings – the part of Earth you’re standing on is facing into the spiral arm of the galaxy to which our sun belongs.

Consider the sky at the opposite time of year. In June, July and August, the evening sky seen from the entire Earth is facing toward the center of the Milky Way galaxy. The galaxy is about 100,000 light-years across, and its center is some 25,000 to 28,000 light-years away. We don’t see into the exact center of the Milky Way, because it’s obscured by galactic dust. But during those Northern Hemisphere summer months (Southern Hemisphere winter months), as we peer edgewise into the galaxy’s disk, we’re gazing across some 75,000 light-years of star-packed space (the distance between us and the center, plus the distance beyond the center to the other side of the galaxy).

Thus – on June, July and August evenings – we’re looking toward the combined light of billions upon billions of stars.

On June, July and August evenings, we look toward the galaxy’s center as indicated by the red arrows. On December, January and February evenings, we look away from the center, as indicated by the blue arrows. Other features, including our galaxy’s primary spiral arms and the sun’s location in the Orion Spur, are also shown. Artist’s illustration via NASA/JPL/Caltech/R.Hurt.

On December, January and February evenings, we’re looking the opposite way – into thesuburbs of the galaxy and into the spiral arm of the galaxy in which our sun resides.

There really are some gigantic stars located in this direction, and they are relatively close to us – in our own neighborhood, so to speak, our own spiral arm.

And that’s why, while the combined light of so many distant stars visible on June, July and August evenings gives the sky a hazy quality, the evening sky in December, January and February looks clearer and sharper.

Brightest star Sirius on left, with constellation Orion, much as you would see them ascending on a January evening in the Northern Hemisphere. Notice the short, straight row of three medium-bright stars: Orion’s Belt. The stars above and below the Belt are Betelgeuse and Rigel. Photo from EarthSky Facebook friend Susan Jensen in Odessa, Washington. Thank you, Susan!

Our local Orion Arm of the Milky Way galaxy. Notice Orion’s Belt stars again, and Orion’s brightest stars Betelgeuse and Rigel. All are members of the Orion Arm. If you visit this page on Wikipedia, you’ll find this image in interactive form.

Our spiral arm of the galaxy is called the Orion Arm, or sometimes the Orion Spur. It’s not one of the primary spiral arms of the Milky Way but only a “minor” spiral arm. Our local Orion Arm is some 3,500 light years across. It’s approximately 10,000 light years in length. Our sun, the Earth, and all the other planets in our solar system reside within this Orion Arm. We’re located close to the inner rim of of this spiral arm, about halfway along its length.

The Orion Arm is also sometimes called the Local Arm, the Orion-Cygnus Arm, or the Local Spur.

Perhaps you know the bright stars of the prominent constellation Orion the Hunter? This constellation is visible in the evening during Northern Hemisphere winter (Southern Hemisphere summer). The stars of mighty Orion also reside within the Orion Arm of the Milky Way. In fact, our arm of the galaxy is named for this constellation.

Northern Hemisphere winter night sky photo by EarthSky Facebook friend Stacy Oliver Bryant. Thank you Stacey! If you look closely, you can see the faint starlit trail of the winter Milky Way. The edgewise view of the Milky Way in winter is less dramatic, because we are looking opposite the center of the galaxy.